Lithium ion secondary batteries, nickel hydride batteries, and other secondary batteries are increasing in importance as power sources mounted in vehicles that use electricity as a drive power source and as power sources for personal computers, portable electronic devices, and other electrical products. In particular, lithium ion secondary batteries, which are light weight and provide a high energy density, are preferred for vehicle-mounted high-output power sources.
A separator for preventing short-circuiting between the two electrodes is disposed between the positive electrode and the negative electrode in these lithium ion secondary batteries. In addition, due to the impregnation of an electrolyte (electrolyte solution) into the pores in this separator, the separator functions to form an ion conduction path (conduction pathway) between the two electrodes.
Separators having a porous layer constituted of a polyolefin polymer, e.g., polyethylene, polypropylene, and so forth, are conventionally used as separators. These separators also have a so-called shutdown function and operate to prevent temperature increases within the battery. Thus, with such a separator having a porous layer constituted of a thermoplastic polymer, when the polymer's melting point (shutdown temperature) is reached due to an increase in the internal battery temperature due to, for example, overcharging, the polymer melts or softens and the pores are then filled and ion conduction between the two electrodes is stopped. As a result, charge/discharge by the battery is forcibly halted and a further rise in temperature can be prevented. Patent Literature 1 is provided as conventional art for separators.